Low velocity staged combustion for furnace atmosphere control

ABSTRACT

An improved staged combustion method useful with oxy-fuel combustion and in a furnace which contains a charge, wherein substoichiometric combustion and low velocity injection of fuel and primary and secondary oxidant are carried out in an orientation which forms a reducing atmosphere proximate the charge surface.

CROSS-REFERENCE TO RELATED APPLICATION

The present invention claims priority to U.S. provisional patentapplication Ser. No. 60/937,768, filed Jun. 29, 2007, the entirecontents of which are incorporated herein by reference in its entirety.

TECHNICAL FIELD

This invention relates generally to staged combustion within a furnacewhich contains a charge to be heated by heat generated by thecombustion.

BACKGROUND OF THE INVENTION

In many industrial heating processes fired with fuel and oxidant,products of fuel combustion interact or react with furnace charge andoften cause undesirable effects. For example, fuel rich flame impingingover glassmelt in a glass melting furnace is known to cause color changein glass product due to redox change of the glassmelt exposed to thefuel rich flame. In a steel reheat furnace oxide scale is formed duringheating resulting in loss of metal or surface defects. In the directreduction of iron oxide by the process disclosed in U.S. Pat. Nos.6,592,649 and 6,602,320, a mixture of iron ore, coal particles and fluxmaterial is agglomerated into balls and laid on a rotary hearth furnace,heated and reduced to produce iron nuggets. Iron oxide is preheated,reduced by carbon from coal, and melted to form iron nuggets. In themelting zone, the reduced charge material is heated by gas burners to1300 to 1500 C to form nuggets and to separate from slag. In thereduction zone, rapid evolution of CO gas from the iron reductionreaction prevents oxidizing gases (CO2, H2O, and O2) in the furnaceatmosphere from oxidizing the charge material. In themelting-nugget-forming zone, little CO is evolved from the chargematerial and reduced iron nuggets are susceptible for re-oxidation byfurnace combustion products (CO2, H2O and excess O2). Prior art hasdisclosed partially solving the re-oxidation problem by charging extracoal particles in the bed of the charge material to protect the ironnuggets from re-oxidation. After devolatization, a bed of char isformed.

There are drawbacks with this approach. Even if the iron nuggets areformed on a bed of excess coke particles, the top surface of each nuggetis exposed to the furnace atmosphere. The melting process requires asignificant amount of heat which is typically provided by combustion ofnatural gas with air. The reactions of CO2 and H2O with carbon areendothermic and consume heat and increase the consumption of naturalgas. It is desirable to prevent re-oxidation of the iron nuggets.

Nitrogen oxides (NOx) are a significant pollutant generated duringcombustion and it is desirable to reduce their generation in carryingout combustion. It is known that combustion may be carried out withreduced NOx generation by using technically pure oxygen oroxygen-enriched air as the oxidant as this reduces the amount ofnitrogen provided to the combustion reaction on an equivalent oxygenbasis. However, the use of an oxidant having a higher oxygenconcentration than that of air causes the combustion reaction to run ata higher temperature and this higher temperature kinetically favors theformation of NOx.

Staged combustion has been used to reduce NOx generation, particularlywhen the oxidant is a fluid having an oxygen concentration which exceedsthat of air. In staged combustion, fuel and oxidant are introduced intoa combustion zone in a substoichiometric ratio and combusted. Due to theexcess amount of fuel available for combustion, very few of the oxygenmolecules of the oxidant react with nitrogen to form NOx. Additionaloxygen is provided to the combustion zone to complete the combustion ina second downstream stage. Because the secondary oxygen is first dilutedwith furnace gases before it mixes with the unburned fuel, thecombustion in the second stage does not occur at very high temperatures,thus limiting the amount of NOx formed.

Using a deeply staged combustion process the furnace atmosphere near thehearth area can be made either more reducing (U.S. Pat. No. 5,755,818)or more oxidizing (U.S. Pat. No. 5,924,858) by vertically stratifyingthe furnace atmosphere. For the direct reduction of iron, a reducingatmosphere near the hearth area is desirable. Although this technologyhas been used commercially in glass melting furnaces where hearth areasare controlled to have a more oxygen rich atmosphere, the degree ofatmosphere stratification was limited due to the relatively highmomentum required for this method. More recently a technology to fullycontrol the furnace atmosphere by providing an inert protectiveatmosphere (such as nitrogen) in the lower half of a directly firedfurnace was described in U.S. Pat. Nos. 5,609,481, 5,563,903, 5,961,689and 6,572,676. The process was applied for aluminum remelting andreduced dross formation by 80% in a full scale furnace (13 ft wide×23 ftlong×8 ft high). Although the process could be applied in a directreduction furnace to create a reducing atmosphere in the lower half ofthe furnace and oxidizing atmosphere in the upper half of the furnace,the large number of special low velocity burners required for theprocess makes the process more complex to operate. A cost effective andbetter stratification method is desirable for the direct reductionprocess, glass melting furnaces and other industrial furnaces wherecombustion atmosphere interacts with the furnace charge.

In order to carry out effective combustion with oxidant having a higheroxygen concentration than that of air, the fuel and/or oxidant must beprovided into the furnace at a relatively high velocity in order toachieve the requisite momentum. The combustion reactants must have acertain momentum in order to assure adequate mixing of the fuel andoxidant for efficient combustion. The high momentum also causes thecombustion reaction products to more effectively spread throughout thefurnace to transfer heat to the furnace charge. Momentum is the productof mass and velocity. An oxidant having an oxygen concentration whichexceeds that of air will have a lower mass than air on an equivalentoxygen molecule basis. For example, an oxidant fluid having an oxygenconcentration of 30 mole percent will have about 70 percent the mass ofan oxidatively equivalent amount of air. Accordingly, in order tomaintain the requisite momentum, the velocity of the combustionreaction, i.e. the velocity of the fuel and/or oxidant of the combustionreaction, must be correspondingly higher.

Accordingly, it is an object of this invention to provide an improvedstaged combustion method wherein fuel and oxidant combust in acombustion reaction having the requisite momentum, with the charge beingprotected from deleterious contact with combustion reaction productswhile still ensuring good heat transfer from the combustion reaction tothe charge.

BRIEF SUMMARY OF THE INVENTION

One aspect of the present invention is a method for carrying outcombustion comprising:

(A) injecting into a furnace which contains a charge, at a point abovethe charge, fuel and primary oxidant in a substoichiometric ratio notexceeding 70 percent of stoichiometric, said primary oxidant being afluid comprising at least 50 mole percent oxygen, both of said fuel andprimary oxidant being injected into the furnace at a velocity of 100feet per second or less;

(B) combusting fuel and primary oxidant within the furnace to produceheat and combustion reaction products including unburned fuel;

(C) injecting secondary oxidant into the furnace above the injectionpoint of the fuel and primary oxidant, said secondary oxidant being afluid comprising at least 50 mole percent oxygen, at a velocity of 100feet per second or less;

(D) establishing a fuel rich gas layer proximate the charge, said fuelrich gas layer being more reducing to the charge than the secondaryoxidant; and

(E) combusting secondary oxidant with unburned fuel to provideadditional heat and combustion reaction products within the furnace.

Another aspect of the invention is a method for carrying out combustioncomprising:

(A) injecting into a furnace which contains a charge, at a point abovethe charge, fuel and primary oxidant in a substoichiometric ratio notexceeding 70 percent of stoichiometric, said primary oxidant being afluid comprising at least 50 mole percent oxygen, both of said fuel andprimary oxidant being injected into the furnace at a velocity of 100feet per second or less;

(B) combusting fuel and primary oxidant within the furnace to produceheat and combustion reaction products including unburned fuel;

(C) injecting secondary oxidant into the furnace below the injectionpoint of the fuel and primary oxidant, said secondary oxidant being afluid comprising at least 50 mole percent oxygen, at a velocity of 100feet per second or less;

(D) establishing a oxygen rich gas layer proximate the charge, saidoxygen rich gas layer being more oxidizing to the charge than thecombustion reaction products within the furnace; and

(E) combusting secondary oxidant with unburned fuel to provideadditional heat and combustion reaction products within the furnace.

As used herein the term “products of complete combustion” means one ormore of carbon dioxide and water vapor.

As used herein the term “products of incomplete combustion” means one ormore of carbon monoxide, hydrogen, carbon and partially combustedhydrocarbons.

As used herein the term “unburned fuel” means material that comprisesone or more of fuel which has undergone no combustion, products ofincomplete combustion of the fuel, and mixtures thereof.

As used herein the term “stoichiometric” means the ratio of oxygen tofuel for combustion purposes. A stoichiometric ratio of less than 100percent means there is less oxygen present than the amount necessary tocompletely combust the fuel present, i.e. fuel-rich conditions. Astoichiometric ratio greater than 100 percent means there is more oxygenpresent than the amount necessary to completely combust the fuel, i.e.excess oxygen conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cross-sectional representation of one embodimentof the invention wherein the gas layer above the charge is reducing.

FIG. 2 is a simplified cross-sectional representation of one embodimentof the invention wherein the gas layer above the charge is oxidizing.

DETAILED DESCRIPTION OF THE INVENTION

The invention will be described in detail with reference to Figure, inwhich is shown industrial furnace 1 which contains a charge 2. Anyindustrial furnace or one or more zones of an industrial furnace whichis heated by one or more burners may be used in the practice of thisinvention. Examples of such furnaces include a steel reheating furnacewherein the charge is steel, an aluminum melting furnace wherein thecharge is aluminum, a glass melting furnace wherein the charge comprisesglassmaking materials, and a cement kiln wherein the charge comprisescement.

Preferred examples are charges which are either susceptible to oxidationor reduction under the conditions that prevail when combustion isoccurring in the furnace. A particularly preferred example that issusceptible for oxidation is a charge comprising iron in its reducedform, or iron in its reduced form mixed with carbonaceous matter such ascoke or charcoal. A particularly preferred example that is susceptiblefor reduction or redox changes is a charge comprising oxidized moltenglass.

Fuel 6 and primary oxidant 7 are provided into furnace 1 at point 3above charge 2 such as through burner 4. The fuel and primary oxidantmay be injected into furnace 1 separately or together in a premixedcondition. The fuel and primary oxidant may be provided into furnace 1through a plurality of burners. Any suitable oxy-fuel burner may beemployed in the practice of this invention. One particularly preferredoxy-fuel burner for use in the practice of this invention is the fueljet burner disclosed in U.S. Pat. No. 5,411,395 to Kobayashi et al.which is incorporated herein by reference.

The fuel may be any gas or other fluid which contains combustibles whichmay combust in the combustion zone of the furnace. Among such fuels onecan name natural gas, coke oven gas, propane, methane and oil.

The primary oxidant is a fluid having an oxygen concentration of atleast 50 volume percent oxygen, preferably at least 90 volume percentoxygen. The primary oxidant may be commercially pure oxygen having anoxygen concentration of 99.5 percent or more.

The fuel and primary oxidant are provided into furnace 1 at flow ratessuch that the stoichiometric ratio of primary oxygen to fuel is lessthan 70 percent and preferably is within the range of from 5 to 50percent of stoichiometric.

Both of the fuel and primary oxidant are injected into furnace 1 at avelocity of 100 feet per second (fps) or less. Preferably the fuel isprovided at a velocity of 50 to 100 fps. Preferably the primary oxidantis provided at a velocity of 2 to 50 fps. These velocities, low relativeto prior art practices, impart the requisite low momentum to thecombustion reactants. The fuel and primary oxidant combust withinfurnace 1 in a combustion reaction 5 to produce heat and combustionreaction products. Combustion reaction products may include products ofcomplete combustion but, owing to the defined substoichiometric primaryoxygen to fuel ratio, will include unburned fuel. The incompletecombustion of the fuel with the primary oxidant enables the combustionof fuel and primary oxidant to proceed at a substantially lowertemperature than would otherwise be the case, thus reducing the tendencyof NOx to form. The combustion reaction products may also include someresidual oxygen because of incomplete mixing and short residence timeduring the combustion reaction although it is possible that theconcentration of oxygen within the combustion reaction products is zero.

In the embodiment of the invention illustrated in FIG. 1, in order toestablish a reducing gas layer over the charge surface, secondaryoxidant 8 is provided into furnace 1 through lance 10 above point 3.Preferably, in this embodiment the secondary oxidant is injected intothe furnace at a point that is further from the upper surface of thecharge 2 than point 3 is. The secondary oxidant may be provided into thefurnace from a point vertically above the fuel and primary oxidant, orfrom a point offset from the vertical, such as by an angle of up to 45degrees.

In the embodiment of the invention illustrated in FIG. 2, in order toestablish an oxidizing gas layer over the charge surface, secondaryoxidant 8 is provided into furnace 1 through lance 10 below point 3.Preferably, in this embodiment the secondary oxidant is injected intothe furnace at a point that is between the upper surface of the charge 2and point 3. The secondary oxidant may be provided into the furnace froma point vertically below the fuel and primary oxidant, or from a pointoffset from the vertical, such as by an angle of up to 45 degrees.

The secondary oxidant is in the form of a fluid having an oxygenconcentration of at least 50 mole percent, preferably at least 90 molepercent. The secondary oxidant may be commercially pure oxygen.Secondary oxidant 8 is provided into furnace 1 at a velocity of 100 fpsor less, and preferably at a velocity which the range of from 50 to 100fps or even as low as 20 fps to 50 fps. It is important to the practiceof this invention that the oxidant have an oxygen concentrationsignificantly greater than that of air. For a given amount of fuelconsumption, the total volume of gases passed through the furnacelessens as the oxygen concentration of the oxidant increases. This lowervolume flux through the furnace, at the velocities required for thestaged combustion practice of this invention, enables the establishmentof the gas layer proximate the charge having a different compositionthan the contents in the rest of the furnace.

Secondary oxidant gas layer 9 has an oxygen concentration which exceedsthat of the combustion reaction products within combustion reaction 5.Although any suitable oxygen lance may be used to inject the secondaryoxidant into the furnace in the practice of this invention, it ispreferred that the secondary oxidant be injected into the furnace usingthe gas injection lance disclosed in U.S. Pat. No. 5,295,816 toKobayashi et al. which is incorporated herein by reference.

The secondary oxidant is provided into the furnace at a flowrate suchthat, when added to the primary oxidant, establishes a stoichiometricratio with the fuel of at least 90 percent, and preferably within therange of from 100 to 110 percent. When the stoichiometric ratio of theprimary and secondary oxidant to the fuel is less than 100 percent, theremaining oxygen needed to achieve complete combustion of the fuelwithin the furnace may be provided by infiltrating air. Preferably, themomentum ratio of the fuel and primary oxidant stream to the secondaryoxidant stream is about 1.0 although some divergence from unity isacceptable, such as a momentum ratio within the range of from 0.3 to 3.0or less.

Heat generated in combustion reaction 5 radiates to the charge to heatthe charge. This heat radiates from combustion reaction 5 to the chargedirectly or indirectly through complex radiative interactions withsurrounding furnace gases and walls. Very little heat is passed from thecombustion reaction to the charge by convection in high temperaturefurnaces.

In the embodiment of the invention illustrated in FIG. 1, because of theposition at which the secondary oxidant is provided into the furnace,there is formed a relatively reducing gas layer which interacts withcharge 2 in a manner which differs from the interaction which wouldoccur were the furnace atmosphere homogeneous. In the embodiment of theinvention illustrated in FIG. 2, because of the position at which thesecondary oxidant is provided into the furnace, there is formed arelatively oxidizing gas layer which interacts with charge 2 in a mannerwhich differs from the interaction which would occur were the furnaceatmosphere homogeneous.

Downstream of combustion reaction 5 the secondary oxidant and theunburned fuel will mix, such as in region 11 within furnace 1, thusserving to prevent the secondary oxidant from directly interacting(reacting) with the oxidizable components of the charge in theembodiment of the invention illustrated in FIG. 1, or serving to preventthe products of incomplete combustion from directly interacting(reacting) with the reducible components of the charge in the embodimentof the invention illustrated in FIG. 2, to complete the combustion ofthe fuel, and to provide additional heat and combustion reactionproducts within the furnace.

The combustion reaction products in furnace 1 are generally exhaustedthrough a flue port located in the coldest area of the furnace in orderto maximize the furnace fuel efficiency. When the present invention isused in a zone of a furnace with multiple zones, the combustion reactionproducts may be exhausted to the adjacent zone. The elevation of theflue port also influences the degree of furnace atmospherestratification. Preferably the combustion reaction products in furnace 1are exhausted from the furnace from a point not below point 3 where fueland primary oxidant are provided into the furnace, such as from flue 12.

1. A method for carrying out combustion comprising: (A) injecting into afurnace which contains a charge, at a point above the charge, fuel andprimary oxidant in a substoichiometric ratio not exceeding 70 percent ofstoichiometric, said primary oxidant being a fluid comprising at least50 mole percent oxygen, both of said fuel and primary oxidant beinginjected into the furnace at a velocity of 100 feet per second or less;(B) combusting fuel and primary oxidant within the furnace to produceheat and combustion reaction products including unburned fuel; (C)injecting secondary oxidant into the furnace above the injection pointof the fuel and primary oxidant, said secondary oxidant being a fluidcomprising at least 50 mole percent oxygen, at a velocity of 100 feetper second or less; (D) establishing a fuel rich gas layer proximate thecharge, said fuel rich gas layer being more reducing to the charge thanthe secondary oxidant; and (E) combusting secondary oxidant withunburned fuel to provide additional heat and combustion reactionproducts within the furnace.
 2. The method of claim 1 wherein the fueland primary oxidant are injected into the furnace in a substoichiometricratio within the range of from 5 to 50 percent of stoichiometric.
 3. Themethod of claim 1 wherein combustion reaction products are withdrawnfrom the furnace at a point not below the point where fuel and primaryoxidant are injected into the furnace.
 4. The method of claim 1 whereinthe charge comprises oxidizable material.
 5. The method of claim 1wherein the charge comprises iron in its fully reduced state.
 6. Themethod of claim 5 wherein the charge further comprises coke or charcoal.7. The method of claim 1 wherein the secondary oxidant is provided at aflowrate sufficient to provide oxygen into the furnace so that thestoichiometric ratio of the primary and secondary oxidant to the fuelinjected into the furnace is at least 90 percent.
 8. A method forcarrying out combustion comprising: (A) injecting into a furnace whichcontains a charge, at a point above the charge, fuel and primary oxidantin a substoichiometric ratio not exceeding 70 percent of stoichiometric,said primary oxidant being a fluid comprising at least 50 mole percentoxygen, both of said fuel and primary oxidant being injected into thefurnace at a velocity of 100 feet per second or less; (B) combustingfuel and primary oxidant within the furnace to produce heat andcombustion reaction products including unburned fuel; (C) injectingsecondary oxidant into the furnace below the injection point of the fueland primary oxidant, said secondary oxidant being a fluid comprising atleast 50 mole percent oxygen, at a velocity of 100 feet per second orless; (D) establishing a oxygen rich gas layer proximate the charge,said oxygen rich gas layer being more oxidizing to the charge than thecombustion reaction products within the furnace; and (E) combustingsecondary oxidant with unburned fuel to provide additional heat andcombustion reaction products within the furnace.
 9. The method of claim8 wherein the fuel and primary oxidant are injected into the furnace ina substoichiometric ratio within the range of from 5 to 50 percent ofstoichiometric.
 10. The method of claim 8 wherein combustion reactionproducts are withdrawn from the furnace at a point not below the pointwhere fuel and primary oxidant are injected into the furnace.
 11. Themethod of claim 8 wherein the charge comprises oxidizable material. 12.The method of claim 8 wherein the charge comprises molten glass.
 13. Themethod of claim 8 wherein the secondary oxidant is provided at aflowrate sufficient to provide oxygen into the furnace so that thestoichiometric ratio of the primary and secondary oxidant to the fuelinjected into the furnace is at least 90 percent.